This invention relates to a radiation absorbing polymer which has chemically bonded thereto an organic chromophore, a coating composition containing the radiation absorbing polymer and an anti-reflective coating formed from the coating composition and, more particularly, to a radiation absorbing polymer capable of forming a radiation absorbing coating such as an anti-reflective coating useful in manufacturing integrated circuit elements by lithography, a composition containing the radiation absorbing polymer, and a radiation absorbing coating such as an anti-reflective coating formed from the composition.
In the field of manufacturing integrated circuit elements, patterning technology to form finer patterns by lithographic process has made progress and, in recent years, in order to attain a higher degree of integration, the development of patterning technology enabling quarter micron fine patterning has been studied. In such a lithographic process, a photoresist is applied to a substrate, a latent image of a mask pattern is created in the photoresist using a reduction projection exposure apparatus, then the latent image is developed using a proper developer solution to obtain a patterned resist with the desired width and pattern. However, many substrates used in the field of manufacturing integrated circuit elements have such a high reflectivity that, upon exposure, exposing light passing through the photoresist layer is reflected on the surface of the substrates and is again incident into the photoresist layer, which causes the problem that desired patterns are not obtained or that patterns with some defects are formed due to exposure by the reflected light of photoresist areas which are not to be exposed. These are called problems of standing wave or notching. Various techniques have been investigated to solve the problems caused by such reflection. For example, there have been attempted a technique of dispersing a dye having radiation absorption at the same exposure wavelength as in the photoresist, a technique of forming a radiation absorbing coating of an inorganic compound such as titanium nitride according to a CVD method, vacuum evaporation method or the like, a technique of forming a radiation absorbing coating by applying a dispersion or solution of a radiation absorbing dye in an organic polymer solution on to a substrate, and a technique of forming a radiation absorbing coating by applying to a substrate a radiation absorbing polymer having chemically bonded thereto a chromophore. Of the above-described techniques, the technique of dispersing a radiation absorbing dye in a photoresist has the problems of reduction in photoresist sensitivity, thinning of the resist layer during development processing, sublimation of the dye upon baking, and the like. The technique of using an inorganic anti-reflective coating has the various problems of difficulty in accurate control of coating thickness, difficulty of forming a coating with uniform coating thickness, requirement for a special apparatus for conducting vapor deposition, poor adhesion with a resist film, a requirement for separately providing a step of transferring a pattern by dry etching, and the like. Further, the technique of dispersing a radiation absorbing dye in the anti-reflective coating involves the problems of separation of the polymer and the dye from each other upon formation of the anti-reflective coating by spin-coating, elution of the dye into a resist solvent, sublimation of the dye into the resist layer upon baking, and the like. On the other hand, the technique of using a radiation absorbing polymer does not involve such problems, and hence this technique has been noted in recent years. Methods of using radiation absorbing polymers as anti-reflective coatings and materials to be used for the methods are described in, for example, Japanese Laid-open Patent Publication Nos. H6-75378 and H6-118656, WO 9412912, U.S. Pat. Nos. 4,910,122 and 5,057,399, etc. Of the radiation absorbing polymers, those polymers wherein a radiation absorbing chromophore is chemically bonded to the polymer skeleton have recently been considered most promising, and methods of using them and their application, have already been studied. Particularly in the process of using radiation having a wavelength not longer than that of an eximer laser, an anti-reflective coating is considered to be necessary, and it has been desired to provide an anti-reflective coating having good properties.
On the other hand, there exists a requirement that, upon formation of a photoresist coating on a substrate having a large unevenness, an undercoating layer or an intermediate layer is first coated on the substrate to make the surface even for forming a resist image with high dimensional accuracy. Investigation for meeting such a requirement is also necessary.
Formation of resist patterns using a radiation absorbing intermediate coating such as an anti-reflective coating between a photoresist layer and a substrate before forming a resist pattern is conducted as follows. That is, a composition for a radiation absorbing coating such as an anti-reflective coating solution is first coated to a substrate and, after baking the coating to be made insoluble in a resist solvent, the resist coating is formed by a coating method on the radiation absorbing coating, such as the anti-reflective coating, and is then subjected to the processes of exposure, development processing, etc. to form a resist pattern, followed by removing the coating such as the anti-reflective coating in the resist-free areas, by dry etching or the like.
The above-described radiation absorbing polymers wherein a dye is chemically bonded to a skeletal polymer, generally have a low solubility in solvents for resists, and hence solvents different from those used for resists, such as cyclohexanone, are often employed as a solvent for the radiation absorbing polymer. In case that a solvent used for forming a radiation absorbing coating, such as an anti-reflective coating, is different from that for resist, there may arise problems that process steps for forming the anti-reflective coating in manufacturing integrated circuits increase in number and, in some cases, properties of the resist layer themselves are adversely affected. In addition, in the case that the anti-reflective coating and the photoresist layer are formed by using the same coating apparatus and the anti-reflective coating materials are insoluble in the solvent for resist, there arises a problem that the anti-reflective coating material might be precipitated due to the influence of mixing of the resist coating waste and the anti-reflective coating solution. The precipitate thus formed might close up pipes for waste liquor, or might scatter as fine powder, resulting in pattern defects. Further, an additional pipe line for feeding a solvent for washing the backside and periphery of substrate might be required. As is described above, an anti-reflective coating composition containing a low molecular weight dye dispersed in a polymer has also been developed. Such composition, however, often causes unevenness in coating thickness when coated on a surface of a substrate with topography, that is, it provides poor coverage, which should desired to be improved. In addition, in conducting the resist process on a substrate with areas with topography, coating of the resist is, in some cases, difficult or it is difficult to make the thickness of the resist uniform, due to difference in the level of the surface. There is also a requirement on planarization of the substrate surface by a film-forming material to get uniform thickness of the resist coating formed thereon for improving the accuracy of the formed resist pattern, as well as preventing reflection. Thus, it has been desired to provide an anti-reflecting coating material which can provide high performance, which enables one to control coverage properties on a surface of a substrate with topography, which undergoes no change in the properties of the anti-reflective coating upon baking, and which is soluble in a solvent for the resist.
Further, it has eagerly been required to attain higher resolution in the resist process and, therefore, wavelength of radiation for exposing resists is being shifted to shorter wavelengths, and a process of using KrF laser (248 nm) has been put into practice. However, since substrates presently used show high reflectivity for radiation of such short wavelength and since thickness of a resist is being reduced with the enhancement of resolution, a radiation absorbing coating is desired which can well prevent reflection even when used in a thin thickness in view of the dry etching process. Therefore, it is necessary to develop a radiation absorbing material which absorbs well the radiation of the shorter wavelength to be used, which shows no coating defects even when coated in a thin thickness, and which can match with various kinds of resists.
This invention is made to provide a radiation absorbing polymer satisfying these requirements, a composition containing such a radiation absorbing polymer, and a radiation absorbing coating formed by using the composition.
That is, a first object of the present invention is to provide a radiation absorbing polymer which satisfies the above-described various requirements, that is, which shows high solubility in a resist solvent, which can form a radiation absorbing coating such as a conformal anti-reflective coating on a substrate with topography, which, in some cases, can fill up depressions on the surface of a substrate to make it even, which shows a high anti-reflective effect, and which can form a resist pattern with good adhesion to the substrate and a resist layer, good dry etchability, high heat-resistance and excellent resolution.
A second object of this invention is to provide a composition which can form a radiation absorbing coating capable of satisfying the above-described requirements.
A third object of the present invention is to provide a method for forming a radiation absorbing coating capable of satisfying the above-described requirements.
The other object of this invention is to provide a radiation absorbing coating and an anti-reflective coating capable of satisfying the above-described requirements.
As a result of intensive investigations, the inventors have found that a radiation absorbing polymer satisfying the above-described requirements can be obtained by using a monomer having a keto group in its side chain as a recurring unit of the radiation absorbing polymer.
That is, one aspect of the present invention is a radiation absorbing polymer which has absorption at a predetermined wavelength radiation and which contains at least both a recurring unit having a keto group in its side chain and represented by the following general formula (1) and a recurring unit having in its side chain an organic chromophore absorbing a predetermined wavelength radiation and represented by the following formula (2): 
wherein
R1 and R2 independently represent a hydrogen atom, an alkyl group or other organic group, and R3 represents an organic group having at least one carbonyl group; 
wherein
R4 and R5 independently represent a hydrogen atom, an alkyl group, a carboxyl group or other organic group, and Y represents a group having an organic chromophore having an absorption at a predetermined wavelength radiation, said organic chromophore being bonded directly or through a linkage group to the carbon atom constituting the main chain.
Another aspect of the present invention is a composition for radiation absorbing coating containing the above-described radiation absorbing polymer.
A further aspect of the present invention is a method of forming a radiation absorbing coating by applying the composition for radiation absorbing coating on a substrate and baking it.
A still further aspect of the present invention is a radiation absorbing coating and an anti-reflective coating formed according to the above-described method.
The present invention is described in more detail by the following descriptions which, however, should not be construed to limit the scope of the present invention.
First, as is described above, the radiation absorbing polymer of the present invention is a radiation absorbing polymer containing at least both the recurring unit represented by the formula (1) and the recurring unit represented by the formula (2) and absorbing at a predetermined wavelength of radiation. Preferred examples of the recurring unit represented by the formula 1 are those represented by the following formula (3), (4) or (5). In the case that a plurality of keto groups are in the recurring unit represented by the following formula (3), (4) or (5), the radiation absorbing polymer shows improved solubility in solvents usually used for resists, due to the existence of the keto groups and, in case that at least one hydrogen atom is bonded to the methylene group of the recurring unit, there results a hard coating owing to the reaction with a cross linking agent based on the activity of the hydrogen atom. 
wherein
R1 and R2 represent independently a hydrogen atom, an alkyl group or other organic group, R6 represents an organic group containing at least one carbonyl group, X1 represents O, S, NR7 or a straight, branched or cyclic alkylene group containing at least one carbon atom, R7 represents, a hydrogen atom or a substituted or non-substituted, phenyl or cyclic, straight or branched alkyl group. 
wherein
R1, R2, R8, R9 and R10 represent independently a hydrogen atom, an alkyl group or other organic group. 
wherein
R1, R2, R12, R13 and R14 represent independently a hydrogen atom, an alkyl group or other organic group, and R11 represents a divalent group.
As monomers for constituting the recurring units represented by the above-described formula 3 or 4, there are specifically illustrated the following ones: 
As the recurring unit represented by the above-described formula 5, there are illustrated, for example, those wherein R11 represents xe2x80x94OR15Oxe2x80x94 or xe2x80x94NHR15Oxe2x80x94 (wherein R15 represents one of substituted or non-substituted, straight, branched or cyclic alkylene groups or a substituted or non-substituted phenylene group), with those wherein R15 represents an alkylene group such as an ethylene group. As monomers for constituting the recurring units represented by the above-described formula 3 or 5, there are specifically illustrated the following ones: 
On the other hand, the recurring unit represented by the above formula 2 is more specifically exemplified by those represented by the following formula 6 or 7: 
wherein
R4 and R5 represent independently a hydrogen atom, an alkyl group, a carboxyl group or other organic group, and Ar is a chromophore which absorbs the predetermined wavelength radiation and represents one of substituted or non-substituted benzene ring, condensed ring or heterocyclic ring groups bonded directly or through a linkage group to the main chain carbon atom. 
wherein
R4 and R5 represent independently a hydrogen atom, an alkyl group, a carboxyl group or other organic group, X2 represents O, S, NR16 or a straight, branched or cyclic alkylene group containing at least one carbon atom, R16 represents a hydrogen atom or a substituted or non-substituted, phenyl group or cyclic, straight or branched alkyl group and Ar1 is a chromophore which has absorption at a predetermined wavelength radiation and represents one of substituted or non-substituted benzene ring, condensed ring or heterocyclic ring groups bonded directly or through a linkage group to X2.
Monomers used for constituting the recurring units represented by the above formula 6 or 7 are exemplified by the following: 
In addition, the radiation absorbing polymer of the present invention may contain other recurring units than those represented by the formulae (1) and (2) in addition to the recurring units represented by the formulae (1) and (2) in order to impart the polymer high radiation absorbing property, high etching rate, good solubility for a particular solvent, good storage stability, cross-linking property (curability) or other preferred properties. As monomers for constituting such other recurring units, usually acrylates or methacrylates are used for imparting solubility to the resulting polymer, and styrenes are used for increasing Tg. Other specific examples of the other comonomers than those for constituting the recurring units of formulae 1 and 2, which can impart preferred properties, include methyl methacrylate, methyl acrylate, 2-hydroxyethyl methacrylate, ethyl methacrylate, 2-(methacryloyloxy)ethyl methacrylate, butyl methacrylate, t-butyl methacrylate, glycidyl methacrylate, methacrylic acid, acrylic acid, acrylonitrile, acrylamide, hydroxymethylacrylamide, 2-isocyanatoethyl methacrylate, 4-acetoxystyrene, 3-methyl-4-hydroxystyrene, styrene, vinyl chloride, ethyl vinyl ether, butyl vinyl ether, isobutyl vinyl ether, cyclohexyl vinyl ether, methyl vinyl ether, maleic anhydride, maleimide, N-substituted maleimide, vinyl acetate, 2-isocyanatoethyl acrylate, etc. Of these, methyl methacrylate, methacrylic acid, methyl acrylate, 2-hydroxyethyl methacrylate, hydroxymethylacrylamide, butyl methacrylate, t-butyl methacrylate, glycidyl methacrylate, methyl vinyl ether, butyl vinyl ether, etc. are preferred.
Main properties to be imparted by these comonomers are illustrated below. In the case of using together with organic chromophore, radiation absorption is more enhanced by using, for example, 2-isocyanatoethyl acrylate, maleic anhydride, maleimide, N-substituted maleimide, 2-isocyanatoethyl methacrylate, etc. as the comonomers, etching rate is increased by using, for example, methyl methacrylate, methyl acrylate, 2-hydroxyethyl methacrylate, ethyl methacrylate, butyl methacrylate, t-butyl methacrylate, acrylic acid, vinyl chloride, etc., solubility for solvents commonly used as solvents for photoresists such as propylene glycol monomethyl ether acetate (PGMEA) or ethyl lactate is improved by using, for example, 2-(methacryloyloxy)ethyl methacrylate, acrylic acid, 4-acetoxystyrene, 3-methyl-4-hydroxystyrene, ethyl vinyl ether, butyl vinyl ether, isobutyl vinyl ether, cyclohexyl vinyl ether, methyl vinyl ether, vinyl acetate, etc., cross-linking property (curability) is improved by using, for example, 2-isocyanatoethyl acrylate, 2-isocyanatoethyl methacrylate, methacrylic acid, glycidyl methacrylate, hydroxymethylacrylamide, etc., Tg is increased by using, for example, styrene, 3-methyl-4-hydroxystyrene, etc. However, the above-described specific comonomers and properties imparted by them are to be construed as merely illustrative and not limitative at all.
As the monomers constituting the recurring unit having in the side chain thereof an organic chromophore which absorbs a predetermined wavelength radiation, there are illustrated, for example, those wherein a hydroxyl group- or amino group-containing organic chromophore is chemically bonded to the acid anhydride group or a carboxyl group bonded to the main chain.
Molecular weight and proportion of the recurring units of the radiation absorbing polymer in accordance with the present invention can widely be varied but, in the case of using as a material for forming radiation absorbing coating, those which have a molecular weight of from about 1,000 to about 500,000 and contain at least 5 mol %, based on the whole recurring units, of the recurring unit represented by the formula 1 and at least 10 mol % of the recurring unit represented by the formula 2 are preferred. More preferably, each of the recurring units represented by the formulae 1 and 2 are contained in an amount of 15 mol % or more based on the whole recurring units.
The radiation absorbing polymer of the present invention can be used as, for example, a material for bottom anti-reflective coating used in manufacturing integrated circuits by dissolving the polymer in a solvent. In the case of employing ultraviolet or deep ultraviolet ray as an exposure source for manufacturing integrated circuits using the radiation absorbing polymer of the present invention, the polymer preferably has a strong absorption in the wavelength region of from 180 to 450 nm. In order to obtain such a radiation absorbing polymer, proper recurring unit or units are selected from the recurring units represented by the foregoing formula (2) and, if necessary, from the recurring units represented by the formula (1) or from the recurring units other than those represented by the formulae (1) and (2). Use of such properly selected recurring units represented by the formula (2), capable of strongly absorbing radiation of the exposure wavelength, enables the radiation absorbing polymer containing the recurring units to strongly absorb the radiation used for exposure, thereby reflection of the exposure radiation from the substrate is prevented and a defect-free resist pattern is formed.
The composition for radiation absorbing coating of the present invention contains the above-described radiation absorbing polymer and comprises the radiation absorbing polymer and, if necessary, additives dissolved in a proper solvent. As the solvent to be used for the composition of the present invention, any of those that have conventionally been used for forming a coating and can dissolve the radiation absorbing polymer and other additives may be used. As preferred examples of the solvents, there are illustrated xcex3-butyrolactone, cyclohexanone, dimethylacetamide, dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone, ethyl lactate (EL), methoxy propanol (PGME), propylene glycol monomethyl ether acetate (PGMEA), methyl amyl ketone (MAK) or an optional mixture thereof. Of these, xcex3-butyrolactone, cyclohexanone, EL, PGME, PGMEA and a mixed solvent of PGME and PGMEA are particularly preferable solvents.
Concentration of the radiation absorbing polymer in the composition can be varied over a wide range depending upon the purpose of use of the composition and the thickness of the radiation absorbing coating. For example, in the case of use as an anti-reflective coating, it is usually 20% by weight or less
As the additives to be contained in the composition for radiation absorbing coating of the present invention, there are illustrated, for example, conventionally known radiation absorbing compounds, surfactants or silane series leveling agents for improving adhesion to a substrate and enhancing coating properties, and the like. In addition, in order to enhance cross-linking density upon formation of the coating, commonly known cross-linking agents and cross-linking auxiliaries may be added. As cross-linking agents and cross-linking auxiliaries, there are specifically illustrated melamine compounds, substituted urea compounds, acid-generating agents which can generate acid upon being heated or irradiated to thereby accelerate cross linking, bisblocked isocyanates, blocked isocyanates and epoxy group-containing polymers, etc. These cross-linking agents or auxiliaries may be either low molecular weight compounds or polymers. These cross-linking agents or auxiliaries are desirably added in an amount of 0.1 to 50% by weight based on the weight of the radiation absorbing polymer. In the composition for radiation absorbing coating may also be incorporated, if necessary, low molecular weight compounds or polymers other than the polymer of the present invention.
The radiation absorbing coating of the present invention containing the radiation absorbing polymer may be formed on, for example, a substrate by coating on a substrate a composition for radiation absorbing coating obtained by dissolving the radiation absorbing polymer and, if necessary, desired additives in a proper solvent or, in some cases, by conducting the polymer-forming reaction on the substrate to thereby directly form a coating of the reaction product on the substrate.
The radiation absorbing coating composition is coated on a substrate in a proper thickness depending upon its use. In the case of forming, for example, an anti-reflective coating, it is coated on a substrate in a dry thickness of 300 to 5,000 xc3x85, by spin coating, cast coating, roller coating or the like. After the coating procedure, the coating is baked on a hot plate or in an oven to make it insoluble in a resist solvent. The baking is conducted at a temperature of about 90 to 260xc2x0 C., preferably 160xc2x0 C. or above.
A photoresist is applied to the radiation absorbing coating, such as the anti-reflective coating thus formed on the substrate, in a predetermined thickness, then prebaked to form a photoresist layer. As the photoresist, either positive working or negative working photoresists can be used. Typical examples of usable photoresists include a positive working photoresist comprising novolak resin and a quinonediazide type light-sensitive agent, a chemically amplified resist, etc. which, however, are not limitative at all. Solvents for the photoresist include EL, PGME, PGMEA, ketones, etc. Prebaking temperatures vary depending upon the kind of photoresist to be used, but is usually about 30 to about 200xc2x0 C. The radiation for exposure of the photoresist can be selected from among visible light, UV rays, deep UV rays, KrF eximer laser, argon fluoride (ArF) laser (193 nm), X-rays, electron beams, etc. As the radiation absorbing polymer used in radiation absorbing coating to prevent the reflection from the substrate, those polymers which absorb the radiation of wavelengths required for exposure, as has been described hereinbefore, are selected. After exposure, the photoresist is subjected to development with a developer solution after optionally post-exposure baking, a resist pattern thus being formed. The radiation absorbing coating such as the anti-reflective coating is then dry etched using gas plasma such as oxygen plasma to thereby form a defect-free resist pattern that serves to process or treat the substrate. Additionally, as the developer solution, there may be used known developers such as an alkaline aqueous solution which contains a metal hydroxide, an organic amine or the like dissolved therein.
By selecting the processing conditions, the composition for radiation absorbing coating of the present invention may also be used as a coating which functions to prevent reflection of radiation and to prevent adverse mutual action between the substrate and the resist or to prevent the adverse action of materials used in the resist or substances produced upon exposure of the resist on the substrate. Further, it may be used as a coating for planarizing the surface of the substrate on which a pattern has already been formed (substrate having topography) by fill up depressions on the surface before coating a photoresist thereon to thereby enhance uniformity in the thickness of the coating, such as a photoresist to be coated thereon.
Additionally, in the case of using the radiation absorbing coating of the present invention as a coating which planarizes the substrate surface, it is proposed to slightly reduce the glass transition temperature (Tg) of the radiation absorbing polymer to cause some flow upon baking and, after being completely solidified, make the coating insoluble in resist solvents. The slight reduction in Tg may be attained, for example, by slightly reducing the cross-linking ability of the radiation absorbing polymer upon being heated. In order to impart to the polymer the function flat XXXX the surface of the substrate, there are various techniques of, for example, properly selecting this degree of polymerization of the radiation absorbing polymer, concentration of the radiation absorbing polymer in the composition or substituents in the recurring units represented by the formula (1) or (2) and properly selecting the proportion of the recurring units represented by the formulae (1) and (2) in the polymer and a type of comonomer other than those represented by the formula (1) or (2), or properly selecting the type of additives.
The coating composition for radiation absorbing of the present invention is soluble in a solvent for a photoresist. Therefore, it enables one to use the same coating apparatus, the same waste liquor apparatus and the same rinsing solution as those used for the resist. In addition, the anti-reflective coating using the radiation absorbing polymer of the present invention which shows a high absorption of DUV (248 nm) can preferably be used as an anti-reflective coating for chemically amplified resists sensitive to DUV. Further, the radiation absorbing coating of the present invention has such a low dependence upon resists that anti-reflective coating materials are not required to be changed when resists are changed in the manufacture of IC, and therefore, no process changes have to be evalusted, which is extremely advantageous for users. For example, AZ(copyright)-BARLi(copyright) coating manufactured by Clariant Corp., a commercially available anti-reflective coating material, designed for i-line (365 nm) exposure is very soluble in cyclohexanone, but is barely soluble in a resist solvent and has, therefore, the defect that used the same coating apparatus as that for coating resist is difficultly used upon applying the anti-reflective coating composition or upon edge rinsing. In addition, though AZ(copyright)-BARLi(copyright) coating itself absorbs DUV, it has such a resist dependence that, in some cases, footing or undercut is observed in the profile of a resulting resist pattern. The radiation absorbing polymer of the present invention also has the feature that, while it is soluble in a resist solvent, it forms a film insoluble in the resist solvent and, is also, insoluble in an aqueous alkaline developer solution for resists, due to being heated at a proper temperature after coating on a substrate. Therefore, the radiation absorbing coating such as the anti-reflective coating of the present invention never suffers dissolution when a photoresist coating composition is coated thereon or when wet a development processing is conducted after exposure. Still further, the radiation absorbing coating of the present invention has the feature that, when a resist pattern is used as an etching mask, it can easily be removed by dry etching.
Additionally, the radiation absorbing polymer of the present invention may be obtained according to various known synthesizing processes. For example, there is illustrated a process of copolymerizing a monomer having a keto group in the side chain and corresponding to the recurring unit represented by the foregoing formula (1) with a monomer having an organic chromophore and corresponding to the recurring unit represented by the foregoing formula (2). The monomer having an organic chromophore may easily be obtained by known synthesis processes, for example, by converting a hydroxyl group- or amino group-containing organic chromophore compound to its acrylate or acrylamide. As the process for obtaining the radiation absorbing polymer, the above-described copolymerization process is the most popular. However, it is also possible to introduce a radiation absorbing group into a polymer by the reaction between a polymer having a reactive group and an organic chromophore compound having a hydroxyl group, an amino group, or the like.
In the present invention, polymerization may be conducted in a proper solvent using a free radical or ionic reaction initiator. The resulting copolymer may be of various structures such as a random copolymer or a block copolymer. As preferred solvents for the polymerization, there are illustrated toluene, tetrahydrofuran, benzene, dimethylformamide, dimethylsulfoxide, ethyl lactate, propylene glycol monomethyl ether acetate (PGMEA), cyclopentanone, cyclohexanone, butyrolactone, 2-heptanone, ethyl-3-ethoxypropanate, ethylene glycol monoethyl acetate, methyl-3-methoxypropanate, etc. These solvents may be used alone or in combination of two or more of them.
As specific examples of the reaction initiators, there are illustrated 2,2xe2x80x2-azobis(isobutyronitrile) (AIBN), 2,2xe2x80x2-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2xe2x80x2-azobis(2-cyclopropylpropionitrile), 2,2xe2x80x2-azobis(2,4-dimethyl valeronitrile), 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), benzoyl peroxide, t-butyl peroxy benzoate, di-t-butyl diperoxy phthalate, t-butyl peroxy-2-ethyl hexanoate, t-butyl peroxy pivalate, t-amyl peroxy pivalate, butyl lithium, etc. However, initiators are not limited only to these.
These polymers may be separated from the solvent, then again dissolved in a proper solvent to prepare an anti-reflective coating composition or a composition for radiation absorbing coating or, if the solvent used in the synthesis reaction can be utilized as a solvent for the anti-reflection coating composition or composition for radiation absorbing coating, it may directly be used as the anti-reflective coating composition or composition for radiation absorbing coating without separating the polymer, or the reaction solution may directly be applied to a substrate such as a wafer after completion of the reaction. Additionally, the anti-reflective coating composition or composition for radiation absorbing coating is desirably subjected to filtration using, for example, using a 0.5, 0.2 or 0.1 micron (xcexcm) filter to thereby remove insoluble fine particles. The filtered solution may directly be applied to a substrate such as a wafer.
Molecular weight of the thus obtained polymer varies depending upon polymerization time, reaction temperature, concentrations of used monomers and initiators, kind of reaction medium, etc. and can easily be controlled by properly selecting these parameters. Polymers having a narrow molecular weight distribution may be obtained by employing ionic polymerization.
Molar ration of the comonomers in the radiation absorbing polymer is decided based on reaction rate of each monomer and employed reaction conditions. The radiation absorption and refractive index of final polymer to a desired wavelength radiation is quite important in determining whether the polymer can be used as a bottom anti-reflective coating or the like or not. Radiation absorption of the coating is preferably in the range of from 2 to 40 per micron in thickness, with 5 to 25 being more preferred. Copolymers composed of three or more types of comonomers also required to provide such absorption. Too strong or too weak absorption fails to provide favorable results as an anti-reflective coating. Radiation absorbing properties required for anti-reflective coating materials also depend upon radiation absorbing properties and refractive index of the photoresist material to be coated thereon. Most preferably, the refractive index of the anti-reflective coating is the same as that of the photoresist layer to be coated thereon and, if not, the two indexes are preferably as close as possible to each other. Since radiation absorbing properties of the anti-reflective coating materials are determined by the radiation absorbing properties and molar ratio of the chromophore-containing monomer, the proportion of the chromophore-containing monomer in mol % is important for the anti-reflective coating material. In the present invention, this proportion can easily be controlled by adjusting the charging proportion of the chromophore-containing monomer, and a polymer with the desired values can be prepared.